tries suggests that a certain "threshold" level of socioeconomic development may be a precondition for a sustained drop in birth rates. This threshold level differs rather substantially from region to region and from culture to culture, being lowest in the countries of Chinese culture on the eastern rim of Asia and much higher in Moslem cultures and the countries of Latin America.
Age Structure Patterns
Before the decline of mortality in western Europe the combination of high fertility and high mortality produced populations that tended to be relatively young, with an average age of about 25. Five or six children were born during the reproductive period of women, but of these only two or three survived to become adults. During the first stages of the demographic transition, as mortality—especially infant mortality—declined while fertility remained high, the average age of the population became lower and family size increased because larger proportions of children survived. Among the economically advanced nations, the presence of increased numbers of children probably contributed eventually to declines in the birth rate, especially in urban settings in which children tended increasingly to become an economic burden. Larger numbers of surviving children called for much more parental care, attention, and support, and also increased the need for community and national provisions for the young, including schools, health services, and recreation facilities.
As birth rates declined in what are now called the developed countries, the populations grew older and family size again became smaller. The median age of western populations rose to about 30, and the number of children ever born declined to two or three per family. The developing nations today, in contrast, still have predominantly young populations with growing family size because of increased child survival. Therefore, the burden of dependency is relatively high in these countries and increases in per capita income are retarded because the number of nonproducing consumers (both children and adults) is about as large as the number of workers.
Fertility, Mortality, and Changes in Family Size
As life expectancy goes up and death rates decline, the proportion of children less than 15 years old in the population increases, provided that the fertility rates remain constant, because fewer children die. For example, with six live births per woman, the percentage of children goes from 39 percent when life expectancy is 30 years to 45 percent with a life expectancy of 60 years, an increase of about 28 percent in the child/adult ratio. The average number of children under 15 in each family increases in the same proportion. However, the effect of changes in death rates on the proportion of children